Signaling system



553C SE xx:

3 Sheets-Sheet 1 O. J. MURPHY EI'AL SIGNALING SYSTEM Filed Aug. 19, 1943v3 3 qt I1U|D U 1 vi) U l Win n 2 CK x Co ($9 3$9$ 9 302:0

0. J MURPHY INVENTORS a NEWB),

ATTORNEY Jan 23, 1945.

Jan. 23, 1945.

o. J. MURPHY ETAL SIGNALING SYSTEM 3 Sheets-Sheet 2 Filed Au .Fan. 23,1945.

o. J. MURPHY ETAL SIGNALING SYSTEM I Filed Aug; 19, 1943 s Sheets-Sheets mad.

ATTO RN V Patented Jan. 23, 1945 2,367,885 ICE SIGNALING SYSTEM OrlandoJ. Murphy, Summit, and Neal D.-Newby,'

Leonia, N. J assignors to Bell Telephone Laboratories, Incorporated, NewYork, N. Y.,- a; corporation of New York Application August 19, 1943,Serial-No. 499,184

- Claims.

This invention relates to signaling systems and particularly totelephone systems in which voice frequency currents are used fortransmitting sig-- nals.

Objects of the invention are the provision of more reliable signaltransmitting means in systerns employing voice frequency signals andparticularly the provision of signal transmitting means arranged toreduce the possibility of false operations.

This invention is an improved signaling system comprising means fortransmitting signals each of which consists of a plurality of currentimpulses in succession, each of a different frequency, the transmissionof the impulses being effected by the balancing and unbalancing ofvaristor-resistor bridges, one for each frequency under thecontrol ofsignal responsive relays. Means comprising a low-pass filter delays thetransition from balanced to unbalanced and unbalanced to balancedcondition of the bridges responsive to operations of the signalreceiving reays.

A feature of the invention is a signaling system comprising. a signaltransmitter arranged to transmit signals each consisting of an impulseof current of one frequency followed by an impulse of current of adifferent frequency and arranged to control the initiation andtermination of the signal impulses by controlling the balance of'varistor-resistor bridges through which thesignal sources areindividually coupled to the line over which signalsare to betransmitted. The production of transients upon transition from onefrequency to the other is minimized by a condenser-resistance networkconnected across thebridges.

A further feature of theinvention is a signaling system comprisinga-signal transmitter arranged to transmit signals consisting of aplurality of current impulses'each of different frequency, thetransmission of the impulses being controlled by gas-filled tubes incombination with varistor-resistor bridges, the balance of which iscontrolled by signal responsive relays.

The invention and its various feature will be further described byconsideringthe specific embodiments of the invention shown in thedrawings which form a par-tof'this specification. The invention is notlimited in its application to the partcular systemshown but isapplicable generally to any telephone or other signaling systememploying voice frequency signaling.

Referring to the drawings,

Figs. 1 and 2 show in a-toll'ofiice, anoperators position OP, anoutgoing trunk circuit O'Il, a

' two-way trunk circuit TWTI, a-toll selector TSl,

a toll line circuit 'ILCI, a signal transmitter STI and a signalreceiver SRI; all individually associated withatoll'line TLI ,Fig. 1further shows an incoming" trunk" circuit I'I'Iconn'ectedto a set ofterminalsinthe bank of selector TSI; and- Fig. 3 shows another toll linecircuit TLC2,- signal transmitter STZ and signal receiver SR2 adaptedfor use with the trunk and selector circuits shown in Fig. 1.

The system represented in the drawings in cludes a plurality of tolloffices each' of which comprises a toll board with operators positionsand cords for answering call'sincoming from local manual and dialofiices, from toll subscriber's" lines, and from other toll ofiices andfor extending incoming calls to'switching' trunks leading to calledlocal manual and dial offices to intertoll trunks and to called tollsubscribers lines. Out going jacks are connected to trunk to local of--fices, to toll subscribers lines and to intertolltrunks. Answering jackare directly connected to recording trunks, to toll subscribers lines,to

intertoll trunks and to incoming trunk circuits detailed disclosure ofthe cord and operators position circuits provided in each of the tolloffices for interconnectingcalling' and called lines and trunks andcontrolling the completion of toll calls. The outgoingtrunk circuit OTI,incoming trunk circuit ITI, two-way trunk circuit TWTI, and tollselector TSI are all similar to corresponding circuits in theaforementioned King. et al. patent. The operators positions are providedwith dials for use in controlling the operation of incoming" andauxiliary toll route selectors in other toll oflices and for controllingthe operation of selector and connector switches in local or communitydial ofilces.

The signal transmitter STI comprises relays 230 and 231 which areoperatively controlled by relay III] of the outgoing trunk circuit OTIon outgoing calls andcontrolled by relay I24 of twoway trunk circuitTWTI on incoming calls. The signal transmitter ST I further comprisesrelay 250 controlled by relay 230 and relays 232 and 236 controlled byrelay 2 31. Relay 23B is a polarized relay having a biasing-windingwhich normally maintains closure of the right contact. Relay 250 is apolarized relay having. three windings, the topwinding being a biasingwinding, the bottom winding being a lockingrand biasing winding and themiddle winding being an operating winding which is connected loy thedelay network 240 to thecontacts of'relay 23D. Normally the middlewinding of relay 25B is energized in the same direction as the biasingwinding thereby to maintain closure of the right contact of this relay.I When relay 230 o'p'erates, the current through the middle winding. of"relay 250*is re versed thereby to effect closure ofthe left contact ofrelay 256, the reversal being delayed by the network 246 for about .025second. The signal transmitter STI further comprises a source Gl ofsignaling current of frequency Fl, a source G2 of signaling current offrequenc F2, transformers 252 and 251 and rectifiers 253 and 254 fortransmitting current of frequency Fl to toll line TLl and transformers262 and 261 and rectifiers 263 and 264 for transmitting current offrequency F2 to toll lin 'ILl. The rectifiers 253 and 254 and rectifiers2'63 and 264 are so poled that there is normally a direct currentthrough all of these rectifiers which reduces their impedance. Thecircuit for this direct current is traced from the positive pole ofbattery B2 through a simplex circuit path including both upper windingsof transformer 252, rectifier 253 and 254 and both lower windings oftransformer 251 to conductor 266, thence through a simplex circuit pathincluding both lower windings of transformer 261, rectifiers 263 and264, and both upper windings of transformer 262 to th negative pole ofbattery Bl, the negative pole of battery B2 and the positive pole ofbattery Bl being connected to ground. With the impedance of rectifiers253 and 254 reduced, the bridge comprising these rectifiers and theresistors 255 and 256 is unbalanced so as to allow the transmission ofcurrent from source Gl through transformers 252 and 251 to line TLI ifand when the cutofi relay 2l6 of line circuit TLCl is released. With theimpedance of rectifiers 263 and 264 reduced the bridge comprising theserectifiers and resistors 265 and 266 is balanced so as to prevent thetransmission of current from source G2 to line TLI. When relay 256operates as hereinafter described, ground is connected through the topwinding of relay 2% to conductor 25l thereby reducing the direct currentthrough rectifiers 253, 254, 263 and 264 below the critical value so asto increase the impedance of these rectifiers. The bridge comprisingrectifiers 253 and 254 and resistors 255 and 256 is thereby balanced toprevent the further transmission of current from source GI and thebridge comprising rectifiers 263 and 264 and resistors 265 and 266 isthereby unbalanced to allow the transmission of current from source G2to the lin TLI if and when the cut-off relay 2| 6 is released. Theenergizaito-n of the top winding of relay 256 produces an additionalbias which, in combination with the energization of the bottom winding,results in the desired percentage break for dial impulse signals. Thecircuit for operatively energizing the cut-off relay 2l6 is controlledby relays 232 and 236 as hereinafter described.

The signal receiver SRl may be similar to that disclosed in the patentto J. G. Walsh, No. 2,366,- 236, granted December 22, 1942.

Assume now that a call incoming to the operators position OP has beenanswered with answering plug APl of cord CDl, that the calling plua CPIof this cord has been inserted in jack J l of outgoing trunk circuitOTl, the call being one which may be completed through intertoll trunkline TLI. Relay H6 is operated responsive to insertion of plug CPl injack J l in the manner described in the aforementioned patent to King etal. The operation of relay 6 closes a circuit for operating signalrelays 236 and 23l of signal transmitter STI. The operation of relay 236effects a reversal of the current through the middle winding of relay256, but this operation of relay 256 is delayed for about .625 second bythe delay network 246. The operation of relay 23| opens the normallyclosed circuit through the upper winding of relay 236, the closing ofthe right contact of relay 236 being delayed for about .100 second bycurrent charging condenser 235. The operation of relay 23l closes acircuit for operating the slow-to-release relay 232 and relay 232 opensthe normally closed circuit through the winding of cut-off relay 2l6 oftoll line circuit TLCZ, thereby to effect the release of relay 2l6.Since the bridge comprising rectifiers 253 and 254 is unbalanced at thetime relay 2l6 releases, current of frequency Fl is transmitted throughtransformer 251, back contacts of relay 2l6, directionally selectivecoils 2H and repeating coil 2l2 to the conductors of toll trunk lineTLI. When relay 256 closes its left contact responsive to theaforementioned reversal of the current through its middle winding,ground is connected through this contact and the bottom winding of relay256 to conductor 25l thereby reducing the direct current throughrectifiers 253, 254, 263 and 264 below the critical value. whereby theresistance of these rectifiers is substantially increased. The bridgecomprising rectifiers 253 and 254 is thereb balanced to end thetransmission of current of frequency Fl and the bridge comprisingrectifiers 263 and 264 is unbalanced to initiate the transmission ofcurrent of frequency F2 over toll line TLI. The resistor 216 andcondenser 21I bridged across conductors 25l and 26l prevent the changefrom one frequency to the other from occurring rapidly enough to producetransients which might effect the false operation of signal receiver SRlor interfere with the operation of receiver SRI in response to incomingsignals. The operation of relay 236, about .075 second after theoperation of relay 256, connects ground to conductor 2| 6 to effect thereo-peration of cut-off relay 2l6 and thereby end the transmission ofcurrent of frequency F2 to the conductors of toll line TLI. Thus theoperation of relays 236 and 23l effects the transmission of a seizuresignal over toll line TLl consisting of current of frequency Fl forabout .025 second followed immediately by current of frequency F2 forabout .075 second.

When the operator dials the digits of the toll route code, and the localoffice code and called line number if the called line is a dial office,relay I I6 is alternately released and reoperated responsive to eachdial impulse in the manner set forth in the aforementioned King et al.patent. Each release and reoperation of relay H6 effects a correspondingrelease and reoperation of relays 236 and 23l. Each release andreoperation of relay 236 effects a corresponding release and reoperationof relay 256, the delay network 246 being effective to delay theresponse of relay 256 for about .625 second. Each release of relay 23lopens the circuit for operating relay 232; but, being slow in releasing,rela 232 does not release during the response of relay 23l to dialimpulses. The release of relay 23l in response to the first dial impulseof a train closes the circuit through the upper winding of relay 236causing the release of relay 236. The release of relay 236 disconnectsground from conductor 2l6 thereby releasing the cut-off relay 2l6. Therelease of relay 2| 6 again connects the upper winding of transformers251 and 261 to coils 2H and 2l2; and, since relay 256 is operated, thebridge comprising rectifiers 253 and 254 is balanced and the bridgecomprising rectifiers 263 and264 is unbalanced so that current offrequency F2 is transmitted over line TLI 'for about .025 second. Whenrelay 256 releases responsive to a dial impulse, ground is disconnectedfrom conductor 25I thereby unbalancing the bridge comprising rectifiers253 and 254 and balancing the bridge comprising rectifiers 263 and 264so that the transmission of current of frequency F2 is ended andtransmission of current of frequency FI is initiated. When relay 250reoperates responsive to the end of each dial impulse, ground is -re--connected to conductor 251 thereby balancing the bridge comprisingrectifiers 253 and 254 and unbalancing the bridge comprising rectifiers263 and 264 so that the transmission of current of frequency Fl is endedand the transmission of current of frequency F2 is initiated. After therelease of relay 236 in response to the first impulse of a train createdby the dialing of one digit, this relay remains normal until all of theimpulses in the train have been received. When relay 236 reoperates,about .100 second after the reoperation of relay 23! at the end of thelast impulse of a train or dial impulses, it reconnects ground toconductor 2l 6 to effect the reoperation of relay 2H] and thus end thetransmission of current of frequency Fl. In this manner each train ofdial impulses is transmitted over toll line TLI.

If the calling operator operates the ringing key (not shown) relay Illlis momentarily released in the manner described in the aforementionedKing et al. patent thereby effecting a momentary release of relays 236and-23I so as to cause the transmission of a ringing signal comprisingcurrent of frequency F2 followed by .4

frequency F2, the same as for a single dial impulse representing thedigit one.

When the plug CPI of cord CDI is removed from jack J I, the release ofrelay H is effected in the manner set forth in the aforementioned Kinget al. patent. The release of relay IIU causes the release of relays 236and 23I. The

release of relay 23I causes the successive release h of relays 236 and2H] whereby current of frequency F2 is transmitted to line TLI. Whenrelay 250 releases, about .025 second after the release of relay 230,the bridge comprising rectifiers 253 and 254 is unbalanced and thebridge comprising rectifiers 263 and 264 is balanced thereby to end thetransmission of current of frequency F2 and initiate the transmission ofcurrent of frequency Fl. When relay 23'2 releases, about .6 second afterthe release of relay 23I, ground is reconnected to conductor 2l6 therebyreoperating relay 2IIJ to end the transmission of current of frequencyFl. Thus a disconnect signal, consisting of current of frequency F2 forabout .025 second followed by current of frequency Fl for about .6second, is transmitted over line TLI If now a seizure signal is receivedby signal receiver SRI responsive to seizure of the line TLI at itsdistant end, relay 220 is operated in the manner described in theaforementioned Walsh patent. The operation of relay 220 con nects groundto conductor II4 thereby effecting the operation of relays I20, I2I andI22 of twoway trunk circuit TWTI and the operation of the line relay(not shown) of selector TSI in the manner set forth in the Walsh patent.The selector TSI is selectively operated responsive to dial impulsesignals received by signal receiver SRI; and on a call to the incomingtrunk circuit ITI, relay ,I6I is operated to light the answering lamp ALin the manner described in the 7 5 Walsh patent. When the call isanswered, the operation of relay I64 connects ground to conductor I54,thereby operating relay I24 of two- Way trunk circuit TWTI. Theoperation of relay I24 closes the circuit for operating relays 230 and23I of signal transmitter STI whereby an answering or off-hooksupervisory signal, similar to a seizure signal on an outgoing call, istransmitted over toll line TLI to the distant office. If the answeringoperator disconnects, relays I64 and I24 are successively released,causing the release of relays 23D and 23] whereby an on-hook supervisorysignal is transmitted over line TLI, this signal being similar to adisconnect signal on an outgoing call.

If an all-trunks-busy or a called line busy condition is encountered onan incoming call, the alternate operation and release of relay I24 iseffected in the manner set forth in the aforementioned Walsh patent,thereby causing the alternate transmission of off-hook and on-hooksignals over toll line TLI The signal transmitter STI of Fig. 1 may bemodified so as to normally balance both of the varistor-resistor bridgesby reversing the polarity of the elements 263 and 264 and reversing thepolarity of the battery BI. When so arranged and relay 256 is normal,the voltage impressed across the bridges is equal to the voltage ofbattery B2 minus the voltage of the battery BI and is of such a polaritythat the resistance of rectifiers 253 and 254 is low and the resistanceof rectifiers 263 and 264 is high; whereby the bridge comprisingrectifiers 253 and 254 is unbalanced to transmit current of frequency Fland the bridge comprising rectifiers 263 and 264 is balanced ,to preventthe transmission of current of frequency F2. When relay 256 operates,the voltage impressed across'the bridges is that of the battery BI andof such a polarity that the resistance of rectifiers 253 and 254 is highand the resistance of rectifiers 263 and 264 is low; whereby the bridgecomprising rectifiers 253 and 254 is balanced to prevent thetransmission of current of frequency Fl and the bridge comprisingrectifiers 263 and 264 is unbalanced to transznit current of frequencyF2.

Assume now that the outgoing trunk, two-way trunk and toll routeselector circuits of Fig. 1 are associated with the toll line circuitTLC2, signal transmitter ST2, and signal receiver SR2 shown in Fig. 3.Relays 336 and 33I, of signal transmitter STZ are controlled by relay Ill] of outgoing trunk circuit O-TI on calls outgoing over the associatedtoll line TL2 and are controlled by relay I24 of two-way trunk circuitTWTI on calls incoming over line TL2. Relays 332 and 336 of the signaltransmitter STZ, relay 3I6 of the toll line circuit TLC2 and relay 326of the signal receiver SR2 are similar to relays 236, 2I0

and 226 in Fig. 2 and perform like functions. The delay network340performs a like function to that of the network 240 of Fig. 2 and thetransformer 344 and gas-filled tubes 345 and 346 perform like functionsto that of relay 250 of Fig. 2. The transmission of current of frequencyFl from source 31I through transformers 312 and 311 is controlled by thebridge consisting of rectifiers 313 and 314and resistors 315 and 316,and the transmission of current of frequency F2 from source 38I throughtransformers 382 and 381 is controlled by the bridge consisting ofrectifiers 383 and 384 and resistors 385 and 386. With tube 345deenergized, the resistance of rectifiers 313 and 314 is reduced bydirect current in a circuit from the positive pole of battery B2,through resistor 35!, conductor 351, right windings of transformer 312,rectifiers 313 and 314, left windings of transformer 311, conductor 359,to the negative pole of battery BI. The bridge comprising rectifiers 313and 314 is thereby balanced to block the transmission of current fromsource 31 I, When the equipment is placed in service and thereafter uponclosure of the right contact of relay 330, an electromotive force isinduced in the right windings of transformer 344, which becomeseffective about .025 second after closure of this contact, to energizetube 345. The energization of tube 345 connects ground throughresistance 341, to conductor 351 to reduce the direct current throughrectifiers 313 and 314. By reducing the current through rectifiers 313and 314, their resistance is increased to unbalance the bridge and thusrender the bridge effective to transmit current of frequency Fl to thetoll line T12 if and when the cut-off relay 3H) is released. With tube346 deenergized, the resistance of rectifiers 383 and 384 is reduced bydirect current in a circuit from the positive pole of battery B2,through resistor 352, conductor 358, right windings of transformer 382,rectifiers 383 and 384, left windings of transformer 381, and conductor360 to the negative pole of battery Bl. The bridge comprising rectifiers383 and 384 is thereby balanced to block the transmission of currentfrom source 38!. When tube 346 is energized by an electromotive forceinduced in the input windings of transformer 344 responsive to thereversal of current through the left windings of this transformer about.025 second after the operation of relay 330, ground is connected toconductor 358 through resistor 348, to reduce the current throughrectifiers 383 and 384. By reducing the current through the rectifiers383 and 384, their resistance is increased to unbalance the bridge andthus render the bridge effective to transmit current of frequency F2 tothe toll line TL2 if and when relay 3H] is released. Condenser 349 isbridged across the anodes of tubes 345 and 346 to effect thedeenergization of tube 345 when tube 345 becomes energized and to effectthe deenergization of tube 346 when tube 345 becomes energized. Resistor353 and condenser 355 and resistor 354 and condenser 356 serve a similarpurpose to that of resistor 210 and condenser 21! of Fig. 2. Thus thetransmitter STZ effects the transmission of seizure, selective,supervisory, ringing and disconnect signals similar to those abovedescribed as being transmitted by the signal transmitter S'Il.

What is claimed is:

1. In a signaling system, a line and a signal transmitter associatedwith said line, said transmitter comprising a signal responsive relay,2. source of current of one frequency, means including avaristor-resistor bridge connecting said source to said line, anothersource of current, the frequency of said other source being differentfrom said one frequency, means including a varistor-resistor bridgeconnecting said other source to said line, each of said bridges beingeffective only when unbalanced to transmit current from the associatedsource to said line, means including a relay controlling the balance ofsaid tridges, and means including said signal responsive relay foroperating the last-mentioned relay to effect the transmission over saidline of signals consisting of impulses of current from said sources.

2. In a signaling system, a line and a signal transmitter associatedwith said line, said transmitter comprising a signal responsive relay, asource of current of one frequency, means including a varistor-resistorbridge connecting said source to said line, another source of current,the frequency of said other source being different from said onefrequency, means including a varistor-resistor bridge connecting saidother source to said line, each of said bridges being effective whenunbalanced to transmit current from the associated source to said lineand effective when balanced to prevent the transmission of current fromthe associated source to said line, means including a relay controllingthe balance of said bridges, and means including a low-pass networkconnecting the contacts of said signal responsive relay to the windingof the last-mentioned relay for controlling the operation of saidlast-mentioned relay, thereby to effect the transmission of signalsconsisting of impulses of current from said sources corresponding to thesignals received by said signal responsive relay, said network beingeffective to delay the response of said last-mentioned relay tooperations of said signal responsive relay for a predetermined intervalof time.

3. In combination in a signaling system, an incoming signaling circuit,a signal responsive relay in said circuit, an outgoing signalingcircuit, a source of current of one frequency, a source of current ofanother frequency, means including a normally unbalancedvaristor-resistor bridge connecting one of said sources to said outgoingcircuit, means including a normally balanced varistor-resistor bridgeconnecting the other of said sources to said outgoing circuit, each ofsaid bridges being effective to transmit current from the associatedsource to said outgoing circuit When said bridge is unbalanced and toprevent the transmission of current from the associated source to saidoutgoing circuit when said bridge is balanced, means including a relaycontrolling the balance of said bridges, and means including said signalresponsive relay for operating and releasing said last-mentioned relaythereby to effect the transmission of signals consisting of impulses ofcurrent from said sources through said outgoing circuit.

4. In a signaling system, a line over which signals are to betransmitted and a signal transmitter associated with said line andadapted to transmit over said line signals consisting of a plurality ofimpulses of current in succession, each impulse consisting of current ofonly one of two signaling frequencies, said transmitter comprising asignal relay responsive to signals to be transmitted over said line, asource of current of one of said frequencies, a source of current of theother of said frequencies, a normally balanced varistor-resistor bridgeconnecting one of said sources to said line, a normally unbalancedvaristor-resistor bridge connecting the other of said sources to saidline, each of said bridges being effective when balanced to prevent thetransmission of current from the associated source to said line andeffective when unbalanced to transmit current from the associated sourceto said line, a relay controlling the balance of said bridges, and meansincluding said signal relay controlling said last-mentioned relay toeffect the transmission of signals over said line.

5. In a signaling system, a line over which signals are to betransmitted and a signal transmitter associated with said line andadapted to transmit over said line signals consisting of a pluralityofimpulses of current in succession, each impulse consisting of current ofonly one of two signaling frequencies, said transmitter Comprising asignal relay responsive to signals to be transmitted over said line, asource of current of one of said frequencies, a source of current of theother of said frequencies, a normally balanced varistor-resistor bridgeconnecting one of said sources to said line, a normally unbalancedvaristor-resistor bridge connecting the other of said sources to saidline, each of aid bridges being effective when balanced to prevent thetransmission of current from the associated source to said line andeffective when unbalanced to transmit current from the associated sourceto said line, a relay controlling the balance of said bridges, meansincluding said signal relay controlling said last-mentioned relay toeffect the transmission of signals over said line,

and means comprising a condenser-resistance network bridged across saidvaristor-resistor bridges for delaying the transition from one frequencyto the other thereby to minimize the production of transients.

6. In a signaling system, a line and a signal transmitter associatedwith said line, said transmitter comprising a signal responsive relay, asource of current of one frequency, means including a varistor-resistorbridge connecting said source to said line, another source of current,the frequency of said other source being different from the frequency ofsaid one frequency, means including a varistor-resistor bridgeconnecting said other source to said line, each of said bridges beingeffective when unbalanced to transmit current from the associated sourceto said line and effective when balanced to prevent the transmission ofcurrent from the associated source to said line, means comprising twogas-filled tubes controlling the balance of said bridges, and meansconnecting the contacts of said signal responsive relay to said tubes torender one of the tubes conductive when said relay is operated and theother of said tubes conductive when said relay is released.

7. In a signaling system, a line and a signal transmitter associatedwith said line, said transmitter comprising a signal responsive relay, a

source of current of one frequency, means includ ing a varistor-resistorbridge connecting said source to said line, another source of current,the frequency of said other source being different from the frequency ofsaid one frequency, means including a varistor-resistor bridgeconnecting said other source to said line, each of said bridges beingeffective when unbalanced to transmit current from the associated sourceto said line and effective when balanced to prevent the transmission ofcurrent from the associated source to said line, means comprising twogas-filled tubes controlling the balance of said bridges, meansconnecting the contacts of said signal responsive relay to said tubes torender one of the tubes conductive when said relay is operated and theother of said tubes conductive when said relay is released. and meanseffective upon energization of either of said tubes to deenergize theother of said tubes.

8. In a signaling system, a line and a signal transmitter associatedwith said line, said transmitter comprising a signal responsive relay, asource of current of one frequency, means including a varistor-resistorbridge connectin said source to said line, another source of current,the

frequency of said other source being different from the frequency ofsaid one frequency, means including a varistor-resistor bridgeconnecting said other source to said line, each of said bridges beingeffective when unbalanced to transmit current from the associated sourceto said line and effective when balanced to prevent the'transmission ofcurrent from the associated source to said line, means comprising twogas-filled tubes controlling th balance of said bridges, and meanscomprising a low-pass network connecting the contacts of said signalresponsive relay to said tubes to render one of the tubes conductivewhen said relay is operated and the other of said tubes conductive whensaid relay is released, said network being effective to delay for apredetermined interval of time the energization of said tubes responsiveto operations of said signal relay.

9. In a signaling system, a line over which signals are to betransmitted and a signal transmitter associated withsaid line andadapted to transmit over said line signals consisting of a plurality ofimpulses of current in succession, each impulse consisting of current ofonly one of two signaling 0 frequencies, said transmitter comprising asignal relay responsive to signals to be transmitted over said line, asource of current of one of said frequencies, a source of current of theother of said frequencies, a normally balanced varistor-resistor bridgeconnecting one of said sources to said line, a normally unbalancedvaristor-resistor bridge connecting the other of said sources to saidline, each of said bridges being effective when balanced to prevent thetransmission of current from the associated source to said line andeffective when unbalanced to transmit current from the associated sourceto said line, means comprising one gas-filled tube for controlling thebalance of one of said bridges, means comprising another gasfilled tubefor controlling the balance of the other of said bridges, and meansincluding said signal relay for effecting the energization anddeenergization of said tubes.

10. In a signaling system, a line over which signals are to betransmitted and a signal transmitter associated with said line andadapted to transmit over said linesignals consisting of a plurality ofimpulses of current in succession, each impulse consisting of current ofonly one of two signaling frequencies, said transmitter comprising asignal relay responsive to signals to be transmitted over said line, asource of current of one of said frequencies, a source of current of theother of said frequencies, a normally balanced varistor-resistor bridgeconnecting one of said sources to said line, a normally unbalancedvaristor-resistor bridge connecting the other of said sources to saidline, each of said bridges being effective when balanced to prevent thetransmissionof current from the associated source to said line andeffective when unbalanced to trans mit current from the associatedsource to said line, means comprising one gas-filled tube forcontrolling the balance of one of said bridges, means comprising anothergas-filled tube for controlling the balance of the other of saidbridges, means including said signal relay for effecting theenergization and deenergization of said tubes, and means comprising alow-pass network connecting the contacts of said signal relay to saidtubes to delay for a predetermined interval of time the energization ofeach of said tubes responsive to operation or release of said signalrelay.

ORLANDO J. MURPHY. NEAL D. NEWBY.

